1. Field of the Invention
The present invention relates to a semiconductor. More particularly, the present invention relates to a method and apparatus for removing particulate contamination in a tungsten silicide deposition process.
2. Discussion of the Related Art
In providing electrical connections to silicon for microelectronic applications, it is frequently desired to provide a conductive interface between a metallization layer, such as aluminum (Al), and a portion of a silicon substrate or layer to be electrically connected to the aluminum, in order to prevent the direct contact of the aluminum with the silicon. The reason for this is that if aluminum were to be directly deposited onto the silicon, various problems may arise.
One significant problem in depositing aluminum directly on silicon is that the aluminum acts as a P-type dopant, and any migration of aluminum atoms into a silicon region would dope the silicon region with P-type impurities. This is especially significant when the aluminum contacts an N-type silicon region, where the migration of P-type aluminum atoms into the silicon region would result in an undesired rectifying contact.
One way to overcome the problems mentioned above is to form one or more interface layers over the silicon before the deposition of an aluminum metallization layer. This type of process is generally described in U.S. Pat. No. 3,777,364 to Schinella et al., although numerous other patents describe similar processes. In such a process, a refractory metal, such as tungsten, molybdenum, palladium, platinum, or tantalum, is deposited over and reacts with an exposed silicon (or polysilicon) substrate or layer to form a silicide layer. Once this silicidation process is completed, top portion of the deposited refractory metal that has not reacted with the silicon, may then be removed. Aluminum is then deposited over the silicide layer.
The resulting silicide layer formed between the aluminum and silicon acts as a barrier to the aluminum atoms, preventing migration of the aluminum into the silicon. The silicide layer also provides a low resistivity contact between the aluminum and the silicon. In addition, this type of process slightly reduces the step height of the resulting device.
A problem with the above method of forming a silicide layer is that, when tungsten silicide is used, for example, as the interface layer between aluminum and silicon, and a chemical vapor deposition (CVD) process employing tungsten hexafluoride(WF.sub.6) as a reactant gas is used, the high temperatures involved in the CVD process can cause the hot CVD chamber walls to react with the WF.sub.6 gas. This results in a lowering of the deposition rate of the tungsten onto the surface of the wafer.
To avoid these problems, a cold-wall radiantly-heated chemical vapor deposition (CVD) system is preferably used to deposit the refractory metal. In such a system, each wafer is heated by, for example, a broad band light source. As a result, the deposition of the refractory metal onto the wafer to form a barrier layer between the silicon substrate and an aluminum layer will not be limited by any reaction of process gases (e.g., WF.sub.6) with high-temperature chamber walls.
A representative CVD system is shown in FIG. 1. The CVD system includes an Ar-1 gas line 12, an NF.sub.3 cleaning gas line 18, an Ar-2 carrier gas line 20, a WF.sub.6 reaction gas line 24, an Ar-3 carrier gas line 26, an SiH.sub.4 source gas line 32, and an Ar-4 gas line 36. The gas lines are engaged with a gas tank (not shown) containing each gas. The gas lines have valves 38, 40, 42, 46, 50, 102, 104, 106 and 108, and a mass flow controller (MFC) 44, activated by a controller (not shown), to control the flow and flow rate of the various gases.
The gas lines 12, 18, 20, 24, 26, and 32 also include a plurality of filters 48, which filter gas passing through the gas lines 12, 18, 20, 24, 26, 28, and 32. A holding apparatus (not shown), e.g., a chuck, is provided to load the wafers to be processed and is set up in the chamber 10. An apparatus for controlling the pressure, such as a pump 52 and valve 54, is also provided to control the pressure within the chamber 10. Through the SiH.sub.4 source gas line 32, SiH.sub.4 silicon source gas is supplied to the chamber 10.
A different kind of silicon source gas, such as dichlorosilane (SiH.sub.2 Cl.sub.2 or DCS), may also be used in accordance with this process. Thus a gas line 28 for DCS may be formed parallel to the source gas line 32. Such a gas line 28 will also have valves 42, 46, and 50, as well as an MFC 44.
The Ar-1, Ar-2, Ar-3, and Ar-4 gas lines 12, 36, 20, and 26, each provide argon (Ar) gas into the chamber 10. The Ar-1 and Ar-2 gas lines 12 and 20 supply argon gas into the upper part of the chamber 10; the Ar-3 gas line 26 supplies argon gas into the middle part of the chamber 10; and the Ar-4 gas line 36 supplies argon gas into the lower part of the chamber 10.
An argon carrier gas is supplied to the chamber 10 with the SiH.sub.4 source gas and the DCS gas through the Ar-3 carrier gas line 26, which is connected to the SiH.sub.4 gas line 32. The Ar-4 gas line 36 diverges from the Ar-3 carrier gas line 26 to provide a second source of argon gas to the chamber 10. The Ar-4 gas line 36 is constructed to be divided from the Ar-3 carrier gas line 26 prior to when the Ar-3 gas line 26 connects with the SiH.sub.4 source gas line 32 and the DCS gas line 28.
The WF.sub.6 reacfion gas line 24, the Ar-2 carrier gas line 20, and the NF.sub.3 cleaning gas line 18 are all coupled to the Ar-1 carrier gas line 12. An argon (Ar) carrier gas is supplied to the chamber 10 through the Ar-2 carrier gas line 20, and the Ar-1 carrier gas line 12, which is coupled to upper part of the chamber 10. The NF.sub.3 cleaning gas is supplied to the chamber 10 through gas lines 18 and 12.
To ensure the purity of a deposition film, the chamber 10 is conventionally cleaned by forming plasma using NF.sub.3 after the deposition process. Despite the fact that the chamber 10 is cleaned before the deposition process, a contamination problem on deposifion film of the wafer by unknown particulate contaminant continuously happens during deposition process. As a result, residual gas in the chamber is analyzed by a residual gas analyzer (not shown) in order to find any source of particulate contaminant. By making use of the residual gas analyzer, the pressure of each compound of the residual gas is measured.
FIG. 2 is a graph detecting the pressure of gas in the chamber 10 by making use of a residual gas analyzer. The fluctuation of the pressure of SiH.sub.4 silicon source gas is shown in a regular pattern in which the pressure of the SiH.sub.4 rises during the deposition step and remains at a constant level (e.g., 1.times.10.sup.-4 Torr) during the cleaning step. However, the pressure of NF.sub.3 experiences an irregular pattern in some regions such as during the initial deposition step. Especially sudden ascents of the pressure of NF.sub.3 (shown in the circles "A" and "B" in FIG. 2) in the initial deposition step means that some NF.sub.3 gas remains in the gas line connected with the chamber 10 after the cleaning step.
The NF.sub.3 gas is used to clean the chamber 10 by forming the plasma and may remain in the chamber 10 and gas line 12 rather than being completely eliminated from the chamber. In this case, during as initial deposition step, residual NF.sub.3 gas in the gas line 12 will be inserted in the chamber with other gas and will react with SiH.sub.4 to produce SiF.sub.x, which acts as a particulate contaminant during the tungsten silicide deposition process.
It would, therefore, be desirable to provide a method and an apparatus for removing particulate contaminants in a tungsten silicide deposition process by forcing the NF.sub.3 gas to be fully excluded or to remain at a minimum level, so that it has no effect within the chamber 10 between the cleaning step and the deposition step.